This invention relates to a calibration liquid for the determination of blood components by means of ion-selective electrodes.
Such determinations, in particular of pH, sodium, potassium and calcium, are performed in great numbers nowadays, and medical actions are often governed by their results. As such determinations are always comparative, the significance of their results is directly related to the quality of the solutions(s) used for calibrating the ion-selective electrodes. As the electrodes gradually drift away, or at least may do so, frequent calibration is desirable, if not necessary. Even without legal obligation, calibrating the electrodes will be done at least every day in many clinical laboratories.
Thus, a reliable and stable calibration liquid is essential. This was emphasized once more by R. A. Durst at a seminar on blood pH and gases, held in Utrecht on June 5 and 6, 1978 (see the Proceedings thereof, ed. by A. H. J. Maas (Utrecht University Press, March 1979)).
An attempt for such a calibration liquid is known from German patent application No. 3,024,044 and the corresponding U.S. Pat. No. 4,363,633. Herein solutions are described containing, besides sodium, potassium, calcium, chloride and optionally other inorganic cations, such as magnesium, as a buffer a partially neutralized nitrogen-containing organic sulfonic acid, which has the advantage of forming with calcium neither complexes nor precipitates. These buffered calibration solutions are stable, too.
However, they may still cause errors (and occasionally will certainly do so) because of the liquid junction potential which occurs at the junction (transition) between salt bridge and sample solution or calibration solution. The error can easily attain a value of 0.002 volts, corresponding to 8% difference in the measured concentration. Moreover, it is now realized that because of complex formation the activities of the above-mentioned cations are generally less than their concentrations, and this is especially true for calcium. Disregarding this phenomenon will result in lower cathode potentials, which may be misinterpreted as concentrations lower than those that actually exist. This latter phenomenon may result in quite sizable errors.
It is conceivable that blood plasma could function as a calibration liquid; it contains all the elements to be determined in blood and also all the additional components that might influence the measuring results. However, blood plasma does not keep. Even when carefully sterilized (for example, by sterile filtration) it is not stable, since, because of a residual lipase acid is liberated slowly and the pH gradually drifts away to lower values, influencing all other electrode potential readings.